Shoal draft vessel propulsion



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2 Sheets-Sheet 1 v l ,f

May 20, 1941- J. Boer-:RT

SHOAL DRAFT VESSEL'-PROPULSION Filed Nov. 5, 19558 MEL,

2 Sheets-Sheet 2 J. l.. BOGERT sHoAL-DRAFT vEssEL PRoPuLsIoN Filed Nov. 5, 19:58

May 20, 1941.

IVIAHINI'. VHUV'ULDUN,

Patented May 20, 1941 UNITED STATES PATENT OFFICE 1 Claim.

My invention relates to improvements in the propulsion of shoal draft vessels, especially vessels employed in pushing large flotllas of barges up and down rivers having shallow tortuous channels. Here, in the United States, we have some 15,000 miles of navigable inland waterways, and many of these waterways are of but limited depth even in their channels. So the problem is how to best apply power to the propulsion of otillas of cargo-carrying barges, up and down stream in rivers flowing 3 or more miles per hour.

The ordinary totally submerged screw-propeller is too large in diameter for this work, unless located inside the hull of the tow-boat, in a tunnel open at the bottom, but with its top raised above the water-line. When the propeller is rotating the air is exhausted from the tunnel, and thereafter the propeller works completely immersed. In this manner a screw-propeller mounted in a tunnel can be used to propel a` vessel whose draft is less than the diameter of the propeller without the lower blades projecting below the keel. One serious disadvantageous characteristic of the tunnel propeller is the amount of power absorbed by the friction of the water of the propeller raceway with the walls of the tunnel. Experience shows that efficiency falls off rapidly when the diameter of the propeller exceeds the draft of the vessel by 20%.

The stern paddle-wheel does not suffer in propulsive eciency from lack of depth of water, since the immersion of the lower edges of the buckets rarely exceeds 2 to 3 feet. But while the stern wheel is ideal for shallow water as a straight propulsive device, its large diameter and great width make it excessively heavy for the power transmitted, and it ls dependent entirely upon rudders for maneuverability. In this respect it is markedly inferior to twin tunnel screw propellers, one of which may be backing while the other is coming ahead.

Twisting and turning power is the kind of maneuverability that is vitally important in handling tows on rivers, which are usually full of hairpin turns for their channels. Without an abundance of such power, the pusher type of tow-boat is more or less helpless in negotiating such changes in channel direction.

Modern tow otillas, comprising upwards of vessels securely lashed together with the towboat at the rear, several hundred feet in length, must frequently be turned or twisted about their common center of gravity, without forward movement, or the foremost vessels of the tow will be run aground.

Twisting in modern twin tunnel screw practice involves the use of 4 rudders, one forward and one aft of each submerged propeller. Pushing forward with one propeller while backing with the other exerts a turning or twisting moment depending upon the distance apart of the two propellers, which is rarely more than 25 feet. It is the pressure of the streams of water driven by the propellers against their rudders that is most eflicacious in turning the entire tow about its common center of gravity, since the leverage of these rudders is 200 or 300 feet.

The totally submerged screw-propeller has no side thrust of its own: it is utterly dependent upon rudders in the stream of water flowing to or from it for any side thrust on the hull. Such, however, is not the case with the semi-submerged propeller: a right-hand screw turning clockwise has a thrust on the water towards the left, and turning anti-clockwise a thrust on the water towards the right. It is the essence of my invention that I employ twin semi-submerged propellers, one right-hand, the other left-hand; one turning clockwise, the other anti-clockwise, so that while both may be thrusting water astern, their side thrusts may balance or partially balance each other at the will of the steersman. The coarseness or neness of the pitch of the propeller blades will determine the percentage of the power absorbed by the screw which is expended in thrusting the water to right or left. In the case of a completely submerged propeller, above the center of rotation the blades will be thrusting the water to right when below the center of rotation the same blades will be thrusting the water to left; and vice versa. As remarked above, it is the utilization of this lmportant difference between the semi-submerged and the totally submerged propeller that is the basis of my inVention-a characteristic which has never been utilized heretofore in tow-boat practice.

Of course, this characteristic of the semi-submerged propeller involved the use of twin; propellers, turning normally in opposite directions, whose shafts transmit equal torques unless cross currents or cross winds call for starboard or port helms. In other words, pilot-house control of the relative power transmitted to the two propeller shafts relieves the rudder or rudders of their most severe duty.

In lifting the propellers half out of water, naturally the blade area in contact with the water is reduced one half. Now the efficiency of any aquatic propelling instrument depends directly upon the mass oi' water acted on. So the lifting of two ordinary propellers half out of water would materially reduce their elciency: this is common experience with ships in ballast, when their propeller blades break the surface of the water.

To cure this defect I resort to the use of a plurality of propellers on both shafts, or to a propeller hub on each shaft which carries a plurality of rows of blades. While I do not claim to have been the rst to have mounted more than one propeller on a propeller shaft, where the propellers are totally submerged, I do claim to have been the first one to make use of more than one semi-submerged propeller on the same shaft as one half of the propelling equipment of a selfpropelled vessel, thereby securing adequate maneuvering ability without sacrifice of efficiency. Naturally, because the after propeller blades work in water that has already been more or less acted upon by preceding blades, the pitch of each row of blades should increase from forward aft, in order that each separate propeller or each separate row of blades should perform its share of the propulsive work. Propellers with detachable blades are well known, and I contemplate the use of such detachable blades, since they facilitate the replacement of an injured blade without resort to dry-dock, and permit change of pitch.

Mounting more than one propeller on a propeller shaft necessitates the use of an outboard propeller shaft bearing and hull stern framing that will assure the alignment of such bearing with the stern-tube. Furthermore, because propeller vibration in shallow water is serious, the hull stern framing should be designated to reduce this vibration as much as possible.

How I have met the foregoing requirements with improved construction may be gathered from the following description of the drawings which form part of this application.

Fig. 1 is a plain view of a pusher tow-boat equipped with my improvements. In this figure the pilot-house is removed and the upper deck partially cut away to disclose a 12-cylinder Diesel-electric generator, the main propulsion electric motors with their gearing and the propeller shafts with their thrust-blocks.

Fig. 2 is a longitudinal sectional elevation of the hull made along the line A-B in Fig. 1.

Fig. 3 is a vertical longitudinal section of the port propeller to an enlarged scale along the line C-D in Fig. 1.

Fig. 4 shows the normal blade immersion which is characteristic of all my sculling propeller wheels, the surface of the water when the propellers are rotating being tangential to the hubs.

Fig. 5 is a side elevation of one of the propellers in relation to the water line.

In all these gures I is the hull; 2 is the Dieselelectric generator; 3 are the electric propulsion motors, while l' is the gearing which transmits their energy to the propeller shafts 5; 6 are the thrust-blocks and 1 the stern tubes, while 8 are the propellers. 4It is perfectly obvious that there would be distinct advantages inherent in detachable blades for the propellers 8, it being easy then to remove and replace a blade without the necessity of resorting to the facilitates of a drydock. and disturbing the propeller hubs; 9 are threaded nuts on propeller shafts 5. It will be noted that although not shown by the drawings, except at the right in Fig. 4, the diameters of the hub flts on propeller shaft 5, normally made tapering, must be steadily enlarged from aft forward, as the propellers must all be threaded on the propeller-shaft from the after end. I0 is the rudder, shewn located in the mid vertical longitudinal plane of the hull and operated in the usual manner by some form of steering engine I I. There may be more than one rudder, but every additional rudder entails a loss of efficiency through the friction of the water passing over its surface. I2 is the pilot-house wheel controlling the steering engine Il. At each side of the steermans wheel is an electric controller I3 connected by electric conduits I4 with the propulsion motors 3, and also with a common bus-bar receiving current from the electric generator 2.

It must be apparent that it is desirable that the amount of current supplied to either the port or the starboard propulsion motors shall at all times be controllable, and that if there be but a single electric generating unit, whether it be Dieselelectric or steam-electric, that it should become like a power-house, a source of electric energy independent of its distribution. By varying the field strength of the propulsion motors, the number of field cutting conductors on the rotors being unalterable, the number of field lines cut can be varied, and therefore the torque; the relative positions of the controller handles, starboard and port, will determine the relative amounts of power utilized by either the starboard or port propeller wheels.

This absolute control of the relative power developed by the two sets of propeller wheels is an essential element in my invention, since in that way the minimum amount of helm may at all times be carried. A consideration of the small arrows proceeding from the propellers in Fig. 1 will convince one that the horizontal thrusts should balance if no starboard or port helm is to be carried. Since this is fundamental to my invention and does not obtain with totally submerged propellers it must not be overlooked.

I do not limit myself to any particular type of electric transmission, any suitable modification of the well-known Ward-Leonard system might be adopted. It isnt even necessary to resort to electric transmission of power from Diesel-electric or steam-electric generators, provided the relative power of the prime-movers attached to the starboard and port propeller wheels are readily controllable by the steersman. A cross wind or current may make desirable a constant difference in power development of 10, 15 or even more per cent.' A rudder set across the stern of a vessel absorbs power wastefully, and the four rudders common to twin tunnel propellers are serious offenders. The transverse thrust of each set of propeller blades, starboard and port, which is wholly lacking in the totally submerged propeller, makes possible a crab-like movement of the tow-boat to starboard or port, and with its movement turns the entire tow around its common center of gravity. To ensure this latter tow-twisting maneuver, I make use of powerful pusher couplings comprising framed knees I5 carrying capstans SIG, by means of which the tow is securely lashed to the tow-boat. These framed pusher knees are subjected to the most severe stresses, and should be so attached to the hull and framing that their loads are transmitted to the thrust-blocks 6. Of equal importance is suitable hull framing to reduce as much as possible the vibration set up side thrusts of the starboard and port propellers. The hull must resist forces set up in three planes: horizontally fore and aft between the thrust-blocks 6 Hb. MARINE PROPULSION,

and the pusher knees I5; horizontally athwartships at the stern produced by the propellers reactions, and vertically also at the stern, due to the moments of the center of effort of the blades around the axis of the propeller-shaft.

The framing shown in Figs. l, 2 and 3, and numbered from I'I to 28 inclusive, is of importance in the practical application of my improvements to pusher tow-boat construction. Of special irnportance to resist vertical oscillations at the stern are members II, I8, I9, 20, 2|, 22, 23 and 24; and members I1, 24 and 25 to resist horizontal athwartships oscillations. Members I9, 25, 26, 21 and 28 resist hogging and sagging stresses. The hull itself l will need trimming tanks in the bow and stern, 29 and 30, as well as a double bottom 3| to support -the propulsive machinery, and insure notation if and when the bottom is pierced by rocks, stumps or wreckage. Two Starboard and two port propeller shafts, each carrying one or more semi-submerged propellers might be employed, the side thrust of -two semi-submerged propellers opposing the side thrust of the other el Lili two; and provided the relative torque applied to these 4` shafts was controllable by the steersman, the arrangement would carry out the object of my invention.

I claim:

A propelling and steering device for a vessel comprising a frame, a rudder means, parallel propeller shafts, and motive means to drive said shafts; said frame secured to and extending in overl-ranging relationship from the stern of the vessel, means on said frame for supporting said shafts above the water line of the vessel, one at each side of the longitudinal center line of the vessel, propellers on said shafts, means at the outer end of the frame for mounting said rudder on said frame for turning movement between said propellers, and control means associated with said motive means and said rudder for regulating the speed and direction of movement of said propellers, and for controlling the turning movement of the rudder.

JOHN LAWRENCE BOGERT. 

